Global Satellite Navigation Systems (GNSS) such as the Global Positioning System (GPS), Galileo, GLONASS, COMPASS, BeiDou and the like use a constellation of satellites broadcasting timing signals to allow position and velocity information to be calculated. The timing signals are received and correlated by GNSS receivers to determine the time of flight of the received timing signal from each satellite. The time of flight can then be combined with information about the location of the satellites to calculate the position and velocity of the receiver.
The timing signals are broadcast using spread spectrum techniques due to the weak nature of such signals. The timing signals are broadcast from the satellites on predefined frequency bands, for example the L1 band (1559-1591 MHz), the L2 band (1215-1237 MHz), and the L5 band (1164-1192 MHz).
Tracking devices, such as anti-theft tracking devices, are often fitted to vehicles to allow the movement and location of the vehicle to be remotely monitored. This can be to help locate stolen vehicles or other purposes such as ensuring that vehicles are driven in accordance with traffic regulations.
Tracking devices work in different ways. Some theft-prevention trackers transmit a beacon signal which can be detected by a beacon receiver. Such systems require the beacon receiver to be close enough to the vehicle to detect the beacon signal. Such systems may only have a range of a few kilometers and do not allow the absolute location and heading of vehicle to be remotely monitored.
Other more elaborate tracker systems include a GNSS receiver coupled with a cellular network transceiver. The GNSS receiver generates location data which is periodically transmitted by the cellular network transceiver via the cellular network to a party tracking the vehicle. This may be as a SMS (Short Message Service) text message containing location co-ordinates. This arrangement does not require a dedicated beacon receiver to be operated in the vicinity of the tracker and allows the absolute position of the tracker to be monitored wherever there is cellular network coverage.
One way to disrupt the operation of this type of tracker system is to transmit an interfering signal in the vicinity of the GNSS receiver. An interfering signal is transmitted across or within the GNSS frequency band (i.e. the frequency band on which timing signals are transmitted from the satellites) reducing the signal to noise ratio (SNR) of the received satellite signals at the GNSS receiver. This impairs or entirely prevents the GNSS receiver from generating accurate location data and therefore prevents the system from tracking the location of the vehicle. Although illegal in many jurisdictions, devices adapted to emit such interfering signals (“jamming devices”) are relatively inexpensive and readily available. A thief may steal a vehicle and at the same time install a jamming device in the vehicle to prevent it being remotely tracked.
As well as disrupting the operation of tracking devices, the operation of such jamming devices may also interfere with the operation of other nearby GNSS receivers such as those in satellite navigation systems in other vehicles.
Jamming devices could be adapted to broadcast interfering signals on other frequencies such as frequencies on which beacon signals are transmitted, as discussed before, and on frequencies that would interfere with cellular network transmissions.
There is therefore a desire to detect the operation of jamming devices, particularly the operation of a jamming device that may be associated with the theft of a vehicle.